The overall objective of the proposed research is to understand the pathogenesis of Marek's disease (MD) and to determine the means by which various factors affect pathogenesis. Specific objectives include: 1) studies to determine the nature of avirulence in the case of certain strains of MD virus and 2) studies of the character of infection and the host response. Special attention is to be directed to the thymus-dependent immune response and to the antigens (viral, tumor, etc.) toward which significant responses are directed.